In the related application Ser. No. 07/693,287, and in copending application Ser. No. 07/498,205 filed Mar. 23, 1990, ozone bleaching of medium consistency comminuted cellulosic fibrous material slurry (paper pulp) is effected. While the systems illustrated therein are effective, it has now been recognized that the ozone-pulp reaction is even quicker than originally presumed, and under some circumstances a more simplified apparatus may be utilized to take advantage of this quick reaction time.
It has been found that the actual time in which the vast majority (e.g. over 80-90%) of the pulp brightness enhancing reaction of ozone with medium consistency pulp takes place is on the order of about 1-5 seconds. Therefore, as long as the pulp can be maintained intimately mixed with the ozone during this time period (without separation of the gas), for the rest of the reaction period pulp configuration is of little importance. According to the present invention the pulp is maintained in intimate contact with the ozone containing gas during this critical, brightness-enhancing, first time period of about 2-5 seconds by passing the pulp and ozone together at a relatively high velocity, e.g. over about 1 m/s, preferably about 2-5 m/s. Then the pulp velocity is slowed since the pulp flows into a larger diameter conduit, so that the residual products can continue to react, preferably for a second time period of about 0.5-5 minutes, but yet the height of the unit can be kept less than about 100 feet, and avoids turns. The movement of the pulp during these first and second time periods is preferably in a first, continuous, generally vertical, path.
According to one aspect of the present invention, a method of ozone bleaching paper pulp having a consistency of about 6-15% throughout treatment, using a mixer, is provided. The method comprises the steps of continuously and substantially sequentially: (a) Feeding ozone in a carrier gas, under a pressure substantially greater than 1 bar, and paper pulp having a consistency of about 6-15%, to a mixer. (b) Effecting uniform and intimate mixing of the pulp and ozone in the mixer. (c) Passing the intimate mixture of ozone and pulp from the mixer in a first, substantially vertical, path at a first velocity of about 1 m/s or greater (preferably about 2-5 m/s) for a during a first time period of about 1-5 seconds, so that the gas and pulp do not separate during movement at the first velocity during the first time period, and the vast majority of the pulp brightening reaction between ozone and pulp takes place. And, (d) reducing the velocity of the pulp and gas mixture while it continues to move in first path for a second time period, so that reaction can move toward completion, by passing the pulp into a larger cross-sectional area portion of the first path. Step (c) is preferably practiced in a small diameter (e.g. 0.5-2 feet) conduit, while step (d) is a larger diameter conduit (e.g. 1.5-10 times larger), with a transition between them. The amount of ozone in the ozone containing gas led to the mixer is about 3-12%, and the reaction conditions are typical, e.g. about 25-90 degrees C. Step (d) is practiced for about 0.5-5 minutes, and then the pulp is moved in a second path, distinctly different from the first path.
According to another aspect of the present invention, a method is provided in which steps (a) and (b) are the same, and comprising the further steps of: (c) passing the intimate mixture of ozone and pulp from the mixer in a first, substantially vertical, path at a first velocity and for a first time period such that the gas and pulp do not separate during movement at the first velocity, and the vast majority of the pulp brightening reaction between ozone and pulp takes place during the first time period, by moving the pulp upwardly in a small diameter conduit; and (d) slowing down the velocity of the pulp and gas mixture while it continues to move in said first path by moving the pulp upwardly into a large diameter conduit above and in open communication with the small diameter conduit.
According to yet another aspect of the present invention, steps (a) and (b) are the same, and then the following steps are practiced: (c) passing the intimate mixture of ozone and pulp from the mixer in a first path at a first velocity of about 1-5 m/s for a first time period of about 1-5 seconds, so that the gas and pulp do not separate during movement at the first velocity during the first time period, and the vast majority of the brightness increasing reaction between ozone and pulp takes place; (d) slowing down the velocity of the pulp and gas mixture while it continues to move in said first path for a second time period of about 0.5-5 minutes; and (e) moving the pulp in a second path, distinctly different from the first path.
It is the primary object of the present invention to provide a simple yet effective method for ozone bleaching of medium consistency paper pulp. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.